The present invention relates to a gas concentration sensing apparatus using a gas concentration sensor which detects a concentration of a specific component of a measured gas, such as oxygen involved in an exhaust gas emitted from an automotive vehicle engine. More specifically, the present invention relates to a gas concentration sensing apparatus which is capable of adequately controlling a voltage applied to the gas concentration sensor.
There are conventional gas concentration sensors each comprising a solid electrolytic element for detecting a specific component of a measured gas. For example, a limit-current type air-fuel ratio sensor (referred to as A/F sensor) detects an oxygen concentration in an exhaust gas. The A/F sensor produces a current signal responsive to the oxygen concentration in the exhaust gas (i.e., air-fuel ratio), when a voltage is applied to the sensor element. The solid electrolytic element (i.e., sensor element) of the limit-current type sensor may be zirconia. The A.C. characteristics of this limit-current type sensor is stabile in a high frequency region of the application voltage equal to or higher than 1 kHz. When an element resistance is detected, an application voltage has a frequency determined by considering the A.C. characteristics of the solid electrolytic element. In such detection of the element resistance utilizing the sensor AC characteristics, an A.C. impedance of the element is obtained from a variation amount of the application voltage and a variation amount of a corresponding current.
A.C.impedance=voltage variation/current variation
To accurately detect the air-fuel ratio, it is necessary to adequately control the application voltage. To realize this, the applicant has previously proposed, as one of prospective systems, an xe2x80x9cair-fuel ratio detecting apparatusxe2x80x9d in the published Japanese patent application No. 10-185861. According to this air-fuel ratio detecting apparatus, the voltage applied to the A/F sensor is controlled by a microcomputer. Especially, a change speed of the application voltage is variably controlled. More specifically, the change rate of the application voltage is small in a specific region corresponding to the stoichiometric A/F value and its vicinity compared with change rates in other regions. Such settings make it possible to improve sensing accuracy in the air-fuel ratio detecting operation.
When the microcomputer is used, the application voltage to the A/F sensor is controlled in the following manner. The microcomputer generates a digital signal representing an application voltage. This digital signal is converted into an analog signal by a D/A converter. The application voltage, thus converted into the analog signal, is applied to the A/F sensor. When the voltage applied to the A/F sensor shows stepwise changes periodically, a tailing phenomenon appears on an element current (i.e., sensor current) at each timing corresponding to each stepwise change of the application voltage as shown in FIG. 16. This tailing phenomenon is inherently derived from the A.C. characteristics of the sensor element, and significantly worsens the sensing accuracy in the air-fuel ratio detecting operation. The tailing phenomenon can be suppressed by reducing a step height in each change of the application voltage. However, to realize this, the D/A converter needs to have higher or improved resolution. The cost will be forcibly increased.
According to this kind of A/F sensor, its activated state and/or deteriorated state can be known by detecting an internal resistance of the solid electrolytic element (i.e., element resistance). The detection of the internal resistance of the solid electrolytic element is generally performed by interrupting the air-fuel ratio detecting operation. However, it is desirable to reduce such a dormant period. Accordingly, the D/A or A/D converter and the microcomputer must have high-speed processing capability. This also leads to cost increase.
As described above, the microcomputer equipped apparatus causes various inconveniences.
In view of the foregoing problems encountered in the prior art, the present invention has an object to provide a gas concentration sensing apparatus which is capable of effectively suppressing the oscillation of the application voltage and accurately detecting a gas concentration.
In order to accomplish this and other related objects, the present invention provides a first gas concentration sensing apparatus comprising a sensor element including a solid electrolytic member with electrodes provided on opposed surfaces of the solid electrolytic member, a gas concentration sensor for generating a current signal responsive to a concentration of a specific component involved in a measured gas when a voltage is applied between the electrodes of the sensor element, and an application voltage control circuit for feedback controlling the application voltage applied to the gas concentration sensor in response to the current signal, characterized in that a resistance value obtainable from a change rate of the application voltage controlled by the application voltage control circuit in response to the current signal is set to be smaller than an A.C. impedance of the sensor element.
Preferably, an application voltage line is defined on a V-I coordinate for determining the voltage applied to the gas concentration sensor from the application voltage control circuit, and an inclination of the application voltage line is larger than an inclination equivalent to the A.C. impedance of the sensor element in a sensor activated condition.
Furthermore, the present invention provides a second gas concentration sensing apparatus comprising a sensor element including a solid electrolytic member with electrodes provided on opposed surfaces of the solid electrolytic member, a gas concentration sensor for generating a current signal responsive to a concentration of a specific component involved in a measured gas when a voltage is applied between the electrodes of the sensor element, and an application voltage control circuit for feedback controlling the voltage applied to the gas concentration sensor in response to the current signal, characterized in that a change speed of the application voltage is suppressed in a feedback loop of the application voltage control circuit.
Preferably, the change speed of the application voltage produced by the application voltage control circuit is reduced in such a manner that a resistance value obtainable by the application voltage per unit time is smaller than an A.C. impedance of the sensor element.
Preferably, a low-pass filter is provided as a delay means for reducing the change speed of the application voltage controlled by the application voltage control circuit in response to the current signal.
Preferably, an operational amplifier constituting the application voltage control circuit has a slow slew rate so as to serve as a delay means for reducing the change speed of the application voltage controlled by the application voltage control circuit in response to the current signal.
Furthermore, the present invention provides a third gas concentration sensing apparatus comprising a sensor element including a solid electrolytic member with electrodes provided on opposed surfaces of the solid electrolytic member, a gas concentration sensor for generating a current signal responsive to a concentration of a specific component involved in a measured gas when a voltage is applied between the electrodes of the sensor element, and an application voltage control circuit for feedback controlling the application voltage applied to the gas concentration sensor in response to the current signal, characterized in that an overall gain of a feedback loop including the gas concentration sensor and the application voltage control circuit is always equal to or smaller than 1.
In the above-described second and third gas concentration sensing apparatus, it is preferable that an application voltage line is defined on a V-I coordinate for determining the voltage applied to the gas concentration sensor, and an inclination of the application voltage line is equalized to an inclination equivalent to a D.C. resistance of the sensor element of the gas concentration sensor in a sensor activated condition.
In this case, the gas concentration sensing apparatus is preferably incorporated in an air-fuel ratio control system for performing a lean burn control of an internal combustion engine.
In all of the above-described first to third gas concentration sensing apparatus, it is preferable to provide a plurality of application voltage lines different in their inclinations on the V-I coordinate to selectively or continuously use the plurality of application voltage lines in accordance with the temperature of the sensor element.